A novel modified ornithine decarboxylase protein and a use thereof

ABSTRACT

Provided are a novel modified ornithine decarboxylase protein having improved putrescine productivity and a use thereof.

TECHNICAL FIELD

The present disclosure relates to a novel modified ornithine decarboxylase protein and a use thereof.

BACKGROUND ART

Putrescine (or 1,4-butanediamine) is an important raw material for the production of polyamide-4,6 including nylon-4,6, and is mainly produced on an industrial scale by the hydrogenation of succinonitrile, which is produced from acrylonitrile by addition of hydrogen cyanide. The chemical synthesis of this compound requires non-renewable petrochemical products as raw materials, and relatively high temperature and pressure in a multi-step and multi-reactor design, as well as the use of expensive catalyst systems. Furthermore, since these raw materials are highly toxic and flammable, the known chemical synthetic processes are environmentally disadvantageous. Accordingly, as an alternative to the chemical production process, a process of producing putrescine from a renewable biomass-derived carbon source is required. Recently, a biochemical process of producing putrescine by environmentally friendly microorganisms has received much attention. Putrescine is a kind of polyamine which is found in a broad spectrum of organisms ranging from bacteria to animals and plants. The concentration of putrescine in E. coli is known to be extremely high, as much as about 2.8 g/L. Also, microorganisms have potentially good resistance to high concentrations of polyamines, and thus they are able to grow and survive in the presence of high concentrations thereof. For example, it has been reported that Corynebacterium glutamicum can grow even in the presence of more than 30 g/L of cadaverine. Accordingly, Studies have been continuously conducted to use microorganisms in the production of industrially available high-concentration polyamines. However, studies on the production of polyamines using microorganisms have not advanced enough to be industrially applicable. Therefore, it is a need to develop a strain capable of producing polyamines in a high yield (Qian Z G, et al., Biotechnol Bioeng, 104: 651-662, 2009; Schneider J, et al., Appl Microbiol Biotechnol, 88: 859-868, 2010).

Meanwhile, ornithine decarboxylase (ODC) is an enzyme found in most microorganisms which converts ornithine into putrescine. ODC in E. coli generally forms a homodimer, and active sites are formed at the dimer interface. The reaction mechanism of ODC requires pyridoxal phosphate (PLP) as a cofactor, and PLP forms a Schiff base at a lysine residue of the active site of the enzyme, which is later displaced by a substrate ornithine that undergoes decarboxylation. When putrescine is produced, ODC again forms a Schiff base with PLP.

When ODC introduced into a putrescine-producing strain, genus Corynebacterium, is a protein encoded by E. coli speC gene, and its activity is reported to be very low. Therefore, in order to develop a strain producing putrescine in a high yield, improvement of ODC, which is an enzyme involved in the final step of the putrescine biosynthetic pathway, is very important. Until now, mutation researches have been performed only to the structure or reaction mechanism of an ODC protein, and there have been no reports regarding an increase activity thereof.

DISCLOSURE Technical Problem

The present inventors have made many efforts to improve an ODC protein, which plays an important role in the production of putrescine but shows low activity. As a result, they have discovered a novel mutation site and introduced a mutation on the site to prepare a modified ODC protein having improved putrescine-producing activity, and they have found that when the modified ODC protein is introduced into a putrescine-producing microorganism, the microorganism is able to produce putrescine in a high yield, thereby completing the present application.

Technical Solution

An object of the present invention is to provide a novel modified ornithine decarboxylase (ODC) protein.

Another object of the present invention is to provide a polynucleotide encoding the modified ODC protein, a vector including the polynucleotide, and a transformant transformed with the vector.

Still another object of the present invention is to provide a method of preparing putrescine, the method including the step of reacting L-ornithine, a mixture containing L-ornithine, or an L-ornithine fermentation liquid with the modified ODC protein.

Still another object of the present invention is to provide a recombinant microorganism which has an improved putrescine-producing activity by changing to the modified ODC protein in a Corynebacterium sp. microorganism having putrescine-producing activity.

Still another object of the present invention is to provide a method of producing putrescine, the method including the steps of culturing the Corynebacterium sp. microorganism which has improved putrescine-producing activity by introduction of the modified ODC protein; and recovering putrescine from a culture obtained in the above step.

Advantageous Effects

A modified ornithine decarboxylase protein according to the present invention has putrescine conversion activity which is 21-fold higher than that of a native form. When the modified ornithine decarboxylase protein introduced into a putrescine-producing strain, putrescine productivity is remarkably increased. Therefore, it may be widely applied to efficient mass production of putrescine as an alternative to the known chemical synthetic pathway.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a comparison of putrescine conversion activity between a native E. coli-derived ODC protein and the ODC protein having an I163A or E165A mutation. In detail, pH increases while a conversion reaction occurs, and when the pH increase is examined by phenol red, an increase in absorbance is observed. The ODC protein having I163A or E165A or both of the mutations was found to show superior putrescine conversion activity compared to the native ODC protein.

BEST MODE

In an aspect to achieve the above objects, the present invention provides a novel modified ODC protein, the modified ODC protein having mutation at one or more amino acid residues selected from the group consisting of an isoleucine amino acid residue at position 163 and a glutamic acid amino acid residue at position 165 from an N-terminus of ornithine decarboxylase (ODC) having an amino acid sequence represented by SEQ ID NO: 1.

As used herein, the term “ornithine decarboxylase (ODC)” refers to an enzyme that catalyzes the following reaction which is the first step of the synthesis of a polyamine from ornithine and the last step of the putrescine synthetic pathway. In the production of putrescine using L-ornithine as a substrate, pyridoxal phosphate (PLP) functions as a cofactor.

L-ornithine<=>putrescine+CO₂  [Reaction Scheme]

In the present invention, ornithine decarboxylase (ODC) may be specifically ODC derived from E. coli, and more specifically, ODC having an amino acid sequence represented by SEQ ID NO: 1, which is derived from Escherichia coli.

In the present invention, a method of obtaining ODC (ornithine decarboxylase) may be performed by applying a variety of methods known in the art. For example, ODC may be obtained by gene synthesis technology including codon optimization for obtaining the enzyme in a high yield in E. coli which is generally used in the enzyme expression, and a method of screening useful enzyme resources by bioinformatics based on genome information of the microorganism, but is not limited thereto.

As used herein, the term “modified ODC protein” refers to an ODC protein in which one or more amino acids in the amino acid sequence of the ODC protein are added, deleted, or substituted. Specifically, the modified ODC protein refers to a protein in which the activity thereof is efficiently increased by the modification of a ODC protein compared to that of the wild-type. In the present invention, the modification may be performed using any general method of improving enzymes which is known in the art, without limitation, and the method is exemplified by strategies such as rational design and directed evolution. For example, the rational design strategy may include mutation in an amino acid at a particular site (site-directed mutagenesis), and the directed evolution strategy may include random mutagenesis. Further, natural modification(s) may occur at amino acid residue(s) at position 163 and/or at position 165 of SEQ ID NO: 1 without external manipulation. As used herein, the terms “modified ODC protein”, “ODC mutant”, and “speC mutant” may be used interchangeably.

Specifically, the modified ODC protein of the present invention may have modification(s) of an isoleucine amino acid residue at position 163 and/or a glutamic acid amino acid residue at position 165 from the N-terminus of ornithine decarboxylase (ODC) which is derived from Escherichia coli and has an amino acid sequence represented by SEQ ID NO: 1. For example, the glutamic acid at position 165 may be replaced with alanine, glycine, serine, or valine, or the isoleucine at position 163 may be replaced with alanine, glycine, serine, or valine. Further, the modified ODC protein may have a double modification of the isoleucine at position 163 and the glutamic acid at position 165, in which the isoleucine at position 163 and the glutamic acid at position 165 may be replaced with an amino acid selected from the group consisting of alanine, valine, serine, and glycine, respectively. Specifically, the isoleucine at position 163 and the glutamic acid at position 165 may be replaced with alanine-alanine, alanine-valine, serine-valine, or valine-valine, respectively.

In embodiments of the present invention, when various combinations of mutations on the amino acids at positions 163 and 165 of the wild-type ODC were found to lead to increase putrescine productivity, these positions are suggested to be very important in the preparation of the ODC mutant having improved putrescine productivity. In particular, when the amino acids present at the important mutation sites were replaced with small amino acid residues (alanine, serine, valine, or glycine), putrescine productivity was increased.

Further, the modified ODC protein of the present invention may consist of any one amino acid sequence of SEQ ID NO: 34 to SEQ ID NO: 57, and specifically, any one amino acid sequence of SEQ ID NO: 34 to SEQ ID NO: 42, SEQ ID NO: 45, and SEQ ID NO: 49 and SEQ ID NO: 57, which is an amino acid sequence of the modified ODC protein in which isoleucine or glutamic acid at position 163 or 165, respectively, from the N-terminus is replaced with a small residue. The modified ODC protein may include any polypeptide having a homology of 50%, 60%, 70%, 75%, 80%, 85%, 90%, 95%, 97%, or 99% or higher to the above sequences, as long as it has the above modification and superior putrescine conversion activity to that of the wild-type.

As used herein, the term “homology” refers to a percentage of identity between two polynucleotide or polypeptide moieties. Sequence correspondence from one moiety to another may be determined by a known technique in the art. For example, homology may be determined by aligning the sequence information of two polynucleotide molecules or two polypeptide molecules directly by using a computer program that is readily available and capable of aligning sequence information. In addition, homology may be determined by hybridizing the polynucleotides under the condition for forming a stable double-strand in the homologous regions and then digesting the hybridized strand by a single-strand-specific nuclease to determine a size of a digested fragment.

As used herein, the term “homologous” refers to the correlation between proteins where all grammatical forms and spelling variations include superfamily-derived proteins and other species-derived homologous proteins having a “common evolutionary origin”. Such proteins (and coding genes thereof) have a sequence homology reflected by a high degree of sequence similarity. However, in general use and in the present invention, when the term “homogeny” is modified by an adjective such as “very high”, it refers to a sequence similarity, but not a common evolutionary origin.

As used herein, the term “sequence similarity” refers to the degree of identity or homology among the nucleotide sequences or amino acid sequences of the proteins which may or may not share a common evolutionary origin. In a specific embodiment, when a polypeptide match between two amino acid sequences is least 21% for a fixed length of an amino acid sequence (specifically at least about 50% and most specifically about 75%, 90%, 95%, 96%, 97%, or 99%), those two sequences are “substantially homologous” or “substantially similar”. Substantially homologous sequences may be identified by comparing the sequences using standard software used in the data bank or, for example, by performing a Southern hybridization experiment under the stringent conditions defined for a certain system. A defined condition suitable for hybridization is within the scope of conventional techniques in the art (e.g., see Sambrook et al., 1989, infra).

In a specific embodiment of the present invention, structural analysis of the E. coli-derived ODC protein was performed, and based on the structural information, mutagenesis was performed by a rational design strategy. Mutations (V156, D160, I163, E165, Q691) for widening an entrance region of a path for substrate entry into the active site, and mutations (N153, D309) for stabilizing PLP, which is a cofactor binding to the active site, were designed and prepared (Examples 1 and 2). In detail, when isoleucine, as an amino acid at position 163, and glutamic acid, as an amino acid at position 165 from the N-terminus, were replaced with alanine through a modification of replacing the bulky residue at the entrance region of the path with a small residue, alanine, activity of the ODC protein was found to be remarkably increased (Example 3). Meanwhile, ODC proteins having 6 other types of mutants, V156A, D160A, Q691A, N153D, N153E, and D309E, for PLP stabilization, showed very low activity or little activity compared to the wild-type. Therefore, it can be seen that isoleucine at position 163 and glutamic acid at position 165 of an E. coli-derived ODC protein (SEQ ID NO: 1) are important residues that function to increase the protein activity. The mutations were performed by site-directed mutagenesis using primers given in Table 1 and PCR.

Further, in a specific embodiment of the present invention, isoleucine at position 163 and glutamic acid at position 165 were replaced with other small residues, serine, valine, or glycine, in addition to alanine, to optimize modifications of the corresponding residues (Example 4 and Table 4). The respective amino acid residues at positions 163 and 165 were replaced with glycine (G), serine (S), or valine (V). As a result, when the amino acid residue at position 163 was replaced with serine and the amino acid residue at position 165 was replaced with valine, a kcat/K_(M) value was increased 4.4-fold and 6.9-fold, respectively, compared to the wild-type (Table 5). Based on this result, the two residues were spontaneously changed, and the ODC activity was examined. The activity was increased to 8-fold higher than that of the wild-type by a combination of I163S and E165V, which showed the highest activity on single mutation. One the other hand, the activity was increased to 21.3-fold higher than that of the wild-type by replacement of both of the amino acid residues at positions 163 and 165 to valine (Example 4 and Table 5).

Overall, the increased activities of the ODC enzyme mutants are attributed to an increase in a kcat/K_(M) value due to an increase in a kcat value, rather than a decrease in a K_(M) value. It implied that the structure of ODC enzyme is changed to increase a conversion rate into the product, putrescine, rather than the binding affinity of the substrate for the enzyme, ornithine.

In the present invention, the activity of the ODC enzyme is assayed by using a reaction converting ornithine into putrescine. In detail, when ODC enzyme converts one molecule of ornithine into putrescine, one molecule of water is consumed and one molecule of carbon dioxide and one OH⁻ ion are produced together with putrescine. Therefore, the total pH is increased. When the increased pH is measured at 559 nm using phenol red, a pH indicator, absorbance is increased in proportion to the pH increase during the reaction. This property is used to indirectly measure a production amount of putrescine.

As used herein, the term “ornithine” refers to a basic amino acid which plays an important role in the ornithine cycle, and in particular, L-ornithine is widely found in plants, animals, and microorganisms. In general, ornithine plays an important role in conjunction with the urea cycle in an organism having the ornithine cycle. Further, ornithine may be interconverted to arginine, glutamic acid, and proline in an organism, and it transfers amino groups to α-keto acid and glyoxylic acid. Ornithine is a substrate producing an amine (putrescine) by ornithine decarboxylase, and a polyamine is synthesized therefrom. In the present invention, ornithine may be specifically L-ornithine which may be used as a substrate of ornithine decarboxylase.

As used herein, the term “putrescine” is a substance produced by decarboxylation of ornithine or hydrolysis of agmatine. Putrescine may be found in putrefaction, but also usually found in a normal component in an organism. Putrescine is a polyamine, and functions to constitute ribosomes and to promote cell growth or RNA synthesis. Industrially, putrescine is an important raw material for the production of polyamide-4,6 including nylon-4,6, and studies for its mass production have been continually demanded.

In another aspect, the present invention provides a polynucleotide encoding the modified ODC protein of the present invention.

As used herein, the term “polynucleotide” encompasses DNA and RNA molecules, and a nucleotide as a basic unit of the polynucleotide includes a natural nucleotide as well as an analogue with a modified sugar or base.

In still another aspect, the present invention provides a vector including the polynucleotide encoding the modified ODC protein of the present invention.

As used herein, the term “vector” refers to any carrier for cloning and/or transferring bases to a host cell. A vector may be a replicon to allow for the replication of the fragments combined with other DNA fragments. “Replicon” refers to any genetic unit acting as a self-replicating unit for DNA replication in vivo, that is, replicable by the self-regulation (e.g., plasmid, phage, cosmid, chromosome, and virus). The term “vector” may include viral and non-viral carriers for introducing nucleotides into a host cell in vitro, ex vivo, or in vivo, and it may also include a mini-spherical DNA. For example, the vector may be a plasmid without a bacterial DNA sequence. Removal of bacterial DNA sequences which are rich in CpG area has been conducted to reduce silencing of the transgene expression and to promote continuous expression of a plasmid DNA vector. The term “vector” may also include a transposon or artificial chromosome.

In the present invention, the vector is a vector including the polynucleotide encoding the modified ODC protein of the present invention, and it may be, but is not particularly limited to, a vector capable of replicating and/or expressing the polynucleotide in a eukaryotic or prokaryotic cell including a mammalian cell (e.g., human, monkey, rabbit, rat, hamster, mouse cell, etc.), a plant cell, a yeast cell, an insect cell, or a bacterial cell (e.g., E. coli, etc.). Specifically, the vector may be a vector that is operably linked to a proper promoter to allow expression of the polynucleotide in the host cell, and includes at least one selection marker. More specifically, the vector may be in the form in which the polynucleotide is introduced into a phage, plasmid, cosmid, mini-chromosome, virus, or retroviral vector.

A pET system using a T7 promoter generally used in the art is well known, and various expression systems known in the art may be used, but is not limited thereto. In the present invention, specifically, the vector including the polynucleotide encoding the modified ODC protein may be a pET28a vector.

In a specific embodiment of the present invention, the polynucleotide encoding the site-directed modified ODC protein was inserted into the pET28a vector by PCR. Through this process, the modified ODC (speC)-expressing vectors, pET28a-speC_I163A, pET28a-speC_I163G, pET28a-speC_I163S, pET28a-speC_I163V, pET28a-speC_E165A, pET28a-speC_E165S, pET28a-speC_E165G, pET28a-speC_E165V, pET28a-speC_I163A_E165A, pET28a-speC_I163S_E165V, pET28a-speC_I163A_E165V, and pET28a-speC_I163V_E165V, were prepared, and the mutations were confirmed by sequence analysis.

In still another aspect, the present invention provides a transformant transformed with the vector.

In the present invention, the transformant is not particularly limited, as long as the modified ODC of the present application is able to express by introducing the vector. The transformant may be bacterial cells such as transformed E. coli, Corynebacterium, Streptomyces, Salmonella typhimurium, etc.; yeast cells; fungal cells such as pichia pastoris, etc.; insect cells such as Drosophila, Spodoptera Sf9 cells, etc.; animal cells such as CHO (Chinese hamster ovary cells), SP2/0 (mouse myeloma cells), human lymphoblastoids, COS, NSO (mouse myeloma cells), 293T, Bowes melanoma cells, HT-1080, BHK (baby hamster kidney cells), HEK (human embryonic kidney cells), or PERC.6 (human embryonic retina cells); or plant cells.

In still another aspect, the present invention provides a method of preparing putrescine, the method including the step of reacting L-ornithine, a mixture containing L-ornithine, or an L-ornithine fermentation liquid with the modified ODC protein.

The L-ornithine, modified ODC protein, and putrescine are the same as described above.

In the present invention, a substance reacted with the modified ODC protein for the preparation of putrescine may be L-ornithine, the mixture containing L-ornithine, or the L-ornithine fermentation liquid. The mixture containing L-ornithine refers to a mixture of separately existing L-ornithine and other components, and the L-ornithine fermentation liquid refers to a fermentation liquid in which L-ornithine is produced or its amount is increased during fermentation, and therefore, L-ornithine sufficient for reaction is included, but is not limited thereto.

For example, the method of producing L-ornithine by fermentation and the produced fermentation liquid are disclosed in U.S. Pat. No. 3,668,072, which is herein incorporated by reference (E. coli, ATCC 21104).

In the present invention, the modified ODC protein may be a purified modified ODC protein or a microorganism fermentation liquid containing the modified ODC protein. Specifically, the microorganism used in the preparation of the microorganism fermentation liquid may be a microorganism expressing the modified ODC protein of the present invention, and more specifically, it may be a transformant microorganism transformed with a vector including the polynucleotide encoding the modified ODC protein of the present invention.

In still another aspect, the present invention provides a microorganism having improved putrescine productivity, which is prepared by changing to the modified ODC protein in a Corynebacterium sp. microorganism having putrescine productivity.

As used herein, the term “microorganism” includes all of a wild-type microorganism and a naturally or artificially genetically modified microorganism, and it may be a microorganism having a particular attenuated or reinforced mechanism due to insertion of a foreign gene or reinforcement or attenuation of activity of an endogenous gene.

As used herein, the term “Corynebacterium sp. microorganism having putrescine productivity” refers to a Corynebacterium sp. microorganism which has putrescine productivity naturally or by modification. It is already known that putrescine is included in a culture of a Corynebacterium sp. microorganism. However, its putrescine productivity is too low, and genes or mechanisms involved in the production have not yet been revealed. Therefore, the “Corynebacterium sp. microorganism having putrescine productivity” in the present invention refers to a native strain itself or a Corynebacterium sp. microorganism in which a foreign gene involved in the putrescine production mechanism is inserted or activity of an endogenous gene is reinforced or weakened so as to have improved putrescine productivity.

As used herein, the term “Corynebacterium sp. microorganism” may be specifically Corynebacterium glutamicum, Corynebacterium ammoniagenes, Brevibacterium lactofermentum, Brevibacterium flavum, Corynebacterium thermoaminogenes, Corynebacterium efficiens, etc., but is not limited thereto. More specifically, the Corynebacterium sp. microorganism in the present invention may be Corynebacterium glutamicum of which cell growth and survival are hardly affected even when exposed to a high concentration of putrescine. For example, the Corynebacterium sp. microorganism may be a Corynebacterium glutamicum KCCM11240P (KCCM11138P ΔNCg11469) strain which is modified to have the weakened NCg11469 activity compared to the endogenous activity thereof, thereby having improved putrescine productivity, but is not limited thereto. The KCCM11240P strain is a putrescine-overexpressing strain prepared by deleting a gene encoding NCg11469 in order to block the biosynthetic pathway of N-acetyl putrescine from putrescine, and is disclosed in International Patent Publication No. WO2013/105827.

In a specific embodiment of the present invention, based on the Corynebacterium sp. microorganism (KCCM11240P (KCCM11138P ΔNCg11469)) having improved putrescine productivity by weakening the NCg11469 activity compared to the endogenous activity thereof, a modified strain was prepared by changing the wild-type speC to an ODC I163S/E165V(speC) mutant having increased putrescine conversion activity in the chromosome (Example 6). The modified strain was designated as Corynebacterium glutamicum CC01-0578, and deposited in the Korean Culture Center of Microorganisms (KCCM) on Jun. 10, 2013 with the accession number KCCM11425P under the Budapest Treaty.

In still another aspect, the present invention provides a method of producing putrescine, the method including the steps of culturing the Corynebacterium sp. microorganism which has an improved putrescine-producing activity by changing to the modified ODC protein according to the present invention; and recovering putrescine from a culture obtained in the above step.

The Corynebacterium sp. microorganism may be specifically Corynebacterium glutamicum, and more specifically, a Corynebacterium glutamicum CC01-0578 (accession number: KCCM11425P) strain.

As used herein, the term “culture” refers to culturing of a microorganism under artificially controlled environmental conditions. In the present invention, the method of producing putrescine using the Corynebacterium sp. microorganism may be conducted using a method widely known in the art. Specifically, examples of the culturing method include a batch process and a fed batch or repeated fed batch process in a continuous manner, but are not limited thereto.

The medium used in the culturing must appropriately satisfy the requirements of specific strains. Culture media for the Corynebacterium sp. microorganism are disclosed (e.g., Manual of Methods for General Bacteriology. American Society for Bacteriology. Washington D.C., USA, 1981). As a carbon source in the medium, sugars and carbohydrates such as glucose, sucrose, lactose, fructose, maltose, starch, and cellulose, oils and fats such as soybean oil, sunflower seed oil, castor oil, and coconut oil, fatty acids such as palmitic acid, stearic acid, and linoleic acid, alcohols such as glycerol and ethanol, and organic acids such as acetic acid, etc. may be used. These substances may be used individually or as a mixture. As a nitrogen source, peptone, yeast extract, beef extract, malt extract, corn steep liquor, soybean meal powder, and urea or inorganic compounds such as ammonium sulfate, ammonium chloride, ammonium phosphate, ammonium carbonate, and ammonium nitrate may be used, and these substances may also be used individually or as a mixture. As a phosphorus source, potassium dihydrogen phosphate or dipotassium hydrogen phosphate or the corresponding sodium-containing salt may be used. In addition, the culture medium may include a metal salt such as magnesium sulfate or iron sulfate which is essential for growth, and finally, essential growth-promoting substances such as amino acids and vitamins may be used in addition to the above-mentioned substances. The appropriate precursor may be added to the culture medium. The above substances may be adequately fed into the culture in a batch or continuous manner.

The pH of the culture may be adjusted by a proper basic compound such as sodium hydroxide, potassium hydroxide, or ammonia, or an acidic compound such as phosphoric acid or sulfuric acid. Foaming may be adjusted by an anti-foaming agent such as a fatty acid polyglycol ester. The aerobic condition of the culture may be maintained by introducing oxygen or oxygen-containing gas mixtures (e.g., air). The culturing temperature may be generally 20° C. to 45° C., specifically 25° C. to 40° C. Culturing may be continued until the production of putrescine reaches the desired maximum, and may usually be achieved in 10 hours to 160 hours. Putrescine may be released into the culture medium, or contained in the cell.

The method of producing the putrescine of the present invention includes a step of recovering putrescine from the cell or culture. The method of recovering the putrescine from the cell or culture may be performed using the appropriate known method in the art, for example, centrifugation, filtration, anion exchange chromatography, crystallization, and HPLC, but is not limited thereto.

MODE FOR INVENTION

Hereinafter, the present invention will be described in more detail with reference to Examples. However, these Examples are for illustrative purposes only, and the invention is not intended to be limited by these Examples.

Example 1 Structural Analysis of ODC (Ornithine Decarboxylase) and Design of Mutant Thereof

In general, E. coli is known to have two types of ODC. One is an inducible ODC (speF), of which expression is induced at an acidic pH, and the other is a constitutive ODC (spec) involved in the production of a diamine such as putrescine (Applebaum D M, et al., Biochemistry, 16: 1590-1581, 1977). Of these, spec, which is the constitutive ODC involved in the production of putrescine, was selected as a target gene.

Until now, the ODC structures of Vibrio and Lactobacillus bacteria have been revealed. Of these, E. coli ODC (spec) was predicted to have a structure similar to that of Lactobacillus 30a ODC. Therefore, based on the 3D structure of Lactobacillus ODC, alignment of the amino acid sequence of E. coli ODC (spec) was performed using a GeneDoc program (Momany C, et al., J Mol Biol, 4: 849-854, 1995). As a result of comparing the amino acid sequences, sequence identity between E. coli speC and Lactobacillus 30a ODC was 53% and sequence similarity therebetween was 65%, indicating that the two enzymes are very similar to each other. Therefore, based on the structure of Lactobacillus 30a ODC (PDB ID: 1ORD) provided by RCSB Protein Data Bank, homology modeling of the E. coli speC structure was performed. As a result, overall skeletons of the proteins were almost identical to each other, and amino acid sequences of the active site involved in binding with PLP (pyridoxal phosphate) were also almost identical to each other.

The result of analyzing the structures of the two enzymes showed that ODC exists as a dimer in cells, the active site thereof is formed at the dimer interface, and the entrance region of a path for a substrate entry into the active site is narrow. Therefore, in order to widen the entrance region for effective entry of the substrate into the active site and rapid conversion of a product, a modification to replace the bulky residues at the entrance region with small residues was designed (V156, D160, I163, E165, Q691).

Additionally, for stabilization of the cofactor PLP binding to the active site, a mutation for the residues surrounding the active site was also designed (N153, D309).

Example 2 Cloning and Expression of E. coli ODC (speC) Gene

To express E. coli speC gene, a pET28a (Novagen) vector system generally used in enzyme expression was used. First, the speC gene was amplified by PCR using the chromosome of the wild-type E. coli W3110 as a template and primers given in the following Table 1. A gene fragment obtained by PCR amplification and a vector pET28a were treated with restriction enzymes, NdeI and XhoI (37° C., 3 hours), and then the speC gene fragment was inserted into the pET28a vector by a general ligation method.

TABLE 1 Primer Primer sequence speC_start (NdeI)_5 5′-cagccatatgaaatcaatga-3′ (SEQ ID NO: 2) speC_stop (XhoI)_3 5′-ggtgctcgagttacttcaac-3′ (SEQ ID NO: 3)

The mutation of the speC expression vector (pET28a-speC) thus prepared was confirmed by sequencing analysis.

The target residues in Example 1 were replaced with a small residue alanine, respectively. To stabilize PLP, the each residues surrounding the active site was modified differently according to the position binding with PLP.

PCR was performed using the prepared pET28a-speC vector as a template and primers given in Table 1 and the following Table 2. First, to mutate the speC gene, primary PCR was performed for the forward (5′) and backward (3′) regions with respect to the region to be mutated, and then secondary PCR for ligation of two PCR fragments was performed. For example, in the case of speC V156A, the forward region was amplified by PCR using speC_start (NdeI)_5 (SEQ ID NO: 2) and speC_V156A_3 (SEQ ID NO: 5) as primers, and the backward region was amplified by PCR using speC_V156A_5 (SEQ ID NO: 4) and speC_stop (XhoI)_3 (SEQ ID NO: 3) as primers. Secondary PCR was performed using the two PCR fragments obtained by the primary PCR as a template and speC_start (NdeI)_5 (SEQ ID NO: 2) and speC_stop (XhoI)_3 (SEQ ID NO: 3) as primers. The speC V156A gene finally obtained was inserted into the pET28a vector in the same manner as in the speC gene fragment. Other mutated fragments were also introduced into the pET28a vector by PCR using primers given in Table 2 in the same manner as above, respectively.

Mutations of the speC mutant expression vectors thus prepared (pET28a-speC_V156A, pET28a-speC_D160A, pET28a-speC_I163A, pET28a-speC_E165A, pET28a-speC_Q691A, pET28a-speC_N153D, pET28a-speC_N153E, pET28a-speC_D309E) were confirmed by sequencing analysis.

TABLE 2 Mutation of entrance region speC_V156A-5 5′-gctgacgcaaaattgggcgatctgctta-3′ (SEQ ID NO: 4) speC_V156A_3 5′-ccaattttgcgtcagcgttacacatatc-3′ (SEQ ID NO: 5) speC_D160A_5 5′-attgggcgctctgcttattcatgaagga-3′ (SEQ ID NO: 6) speC_D160A_3 5′-aagcagagcgcccaattttacgtcagcg-3′ (SEQ ID NO: 7) speC_I163A_5 5′-ctgcttgctcatgaaggatcggcgaaag-3′ (SEQ ID NO: 8) speC_I163A_3 5′-ttcatgagcaagcagatcgcccaatttt-3′ (SEQ ID NO: 9) speC_E165A_5 5′-attcatgcaggatcggcgaaagatgcgc-3′ (SEQ ID NO: 10) speC_E165A_3 5′-cgatcctgcatgaataagcagatcgccc-3′ (SEQ ID NO: 11) speC_Q691A_5 5′-gagctggcaggtgtttatagcgaaaccg-3′ (SEQ ID NO: 12) speC_Q691A_3 5′-aacacctgccagctccggcgaaaatccc-3′ (SEQ ID NO: 13) Mutation for PLP stabilization speC_N153D_5 5′-tatgtgtgacgctgacgtaaaattgggc-3′ (SEQ ID NO: 14) speC_N153D_3 5′-gtcagcgtcacacatatcggcgcgaaag-3′ (SEQ ID NO: 15) speC_N153E_5 5′-tatgtgtgaagctgacgtaaaattgggc-3′ (SEQ ID NO: 16) speC_N153E_3 5′-gtcagcttcacacatatcggcgcgaaag-3′ (SEQ ID NO: 17) speC_D309E_5 5′-ctgtttgaatccgcgtgggtcggttatgaa- (SEQ ID NO: 18) 3′ speC_D309E_3 5′-cgcggattcaaacagaatgtaatcacaca-3′ (SEQ ID NO: 19)

Example 3 Measurement of an Activity of ODC (speC) Mutant Enzymes

3-1. Preparation of ODC Mutant Enzymes Each of the pET28a-speC mutant vectors prepared in Example 2 was transformed into E. coli having DE3 gene type to prepare a strain expressing the ODC enzyme.

Expression of the pET28a-speC mutant vector was performed with reference to a pET system manual (Novagen). In detail, single colonies of respective strains were selected from LB plate media and inoculated into 3 mL of LB liquid medium (+kanamycin 50 μg/mL), followed by incubation at 37° C. and 200 rpm for 16 hours. The culture was re-inoculated into 15 mL of fresh LB medium (+kanamycin 50 μg/mL), and incubated under the same conditions until OD₆₀₀ reached about 0.6. Then, IPTG was immediately added at a final concentration of 0.5 mM and incubated at 18° C. and 180 rpm for 20 hours to induce enzyme expression.

After induction of the enzyme expression, the obtained cells were sonicated and centrifuged. The resulting supernatant was used for a primary activity test. Additionally, to characterize the enzyme, the enzyme was purified and then subjected to a secondary activity test. The enzyme was isolated through a Ni-NTA column using His-tag which was linked to the enzyme in the pET vector. In the purification, a Chelating Excellose spin kit (Bioprogen) was used. ODC (wild-type and mutant speC) enzymes thus obtained were expressed in the soluble form through 8% SDS PAGE, and thus recovered from the supernatant.

3-2. Measurement of an Activity of ODC (Spec) Mutant Enzymes

To evaluate putrescine conversion activity by ODC using ornithine as a substrate, activities of ODC (wild-type and mutant speC) enzymes obtained in Example 3-1 were measured. The ODC activity test to examine putrescine conversion activity was performed with reference to criteria previously reported (Vienozinskiene J, et al., Anal Biochem, 146: 180-183, 1985).

That is, when an ODC enzyme converts one molecule of ornithine into putrescine, one molecule of water is consumed and one molecule of carbon dioxide and one OH⁻ ion are produced together with putrescine. Therefore, total pH is increased (Reaction Scheme 1). When the increased pH is measured at 559 nm using phenol red, a pH indicator, absorbance is changed. The absorbance is increased in proportion to the pH increase. An amount of putrescine was indirectly measured by using this property.

L-ornithine+H₂O->putrescine+CO₂+OH⁻  [Reaction Scheme 1]

For a primary activity test of the ODC enzymes, an amount of total protein in supernatants before purification was quantified and concentrations of supernatants were adjusted equally. The reaction solution was prepared using 30 μg of enzyme supernatant, 10 mM ornithine, and 1.25 μM PLP, and then 40 μM phenol red was used to monitor pH change.

As a result of the activity measurement, the activity of I163A and E165A of the ODC mutant enzymes showed a higher putrescine production rate than that of the wild-type. The activity of remaining 6 types of ODC mutants, V156A, D160A, Q691A, N153D, N153E, and D309E, showed little changes in absorbance at 559 nm (see FIG. 1).

To characterize the two ODC mutant enzymes, I163A and E165A, which were selected in the primary screening, these were purified with His-tag and quantified, and then a rate of putrescine conversion according to ornithine concentration was measured. The ODC enzyme was used at a concentration of 10 μg, and ornithine was used at a concentration of 0.15 mM to 10 mM. In this range, pH change was measured using phenol red.

TABLE 3 kcat/K_(M) ODC enzyme K_(M) (mM) kcat (sec⁻¹) (sec⁻¹M⁻¹) Fold (kcat/K_(M)) WT (wild-type) 1.5 1.6 1.1 × 10³ 1.0 I163A mutant 0.7 1.8 2.6 × 10³ 2.4 E165A mutant 1.1 2.4 2.2 × 10³ 2.0

These results showed that the modified ODC enzymes, I163A and E165A, designed through the ODC structural analysis showed 53% and 27% reductions in the K_(M) value compared to the wild-type, respectively, indicating that their binding affinities for the substrate ornithine were increased. Further, the activity of the modified ODC I163A and E165A showed 12.5% and 50% increases in the kcat value compared to WT, respectively, indicating that the ability to convert ornithine into putrescine was also increased. Finally, the kcat/K_(M) value showing the characteristic of the enzyme activity was calculated. I163A and E165A showed 2.4- and 2-fold increases in the kcat/K_(M) value compared to WT, respectively (Table 3).

Example 4 Optimization of ODC (speC) Mutation

As confirmed in Example 3, mutations of a variety of small amino acid residues at the amino acid (isoleucine) at position 163 and the amino acid (glutamic acid) at position 165, which are important residues in ODC activity, were performed. The mutations was performed in the same manner as in Example 1, and primers used therein are given in the following Table 4. Additionally, single mutations were performed into the positions 163 and 165, respectively and then double mutants were prepared by introducing each position with a mutation combination showing increased ODC activity, followed by evaluation.

TABLE 4 Primer Primer sequence speC_I163G_5 5′-ctgcttggtcatgaaggatcggcgaaagat- (SEQ ID NO: 20) 3′ speC_I163G_3 5′-ttcatgaccaagcagatcgcccaatttt-3′ (SEQ ID NO: 21) speC_I163S_5 5′-ctgctttctcatgaaggatcggcgaaagat- (SEQ ID NO: 22) 3′ speC_I163S_3 5′-ttcatgagaaagcagatcgcccaatttt-3′ (SEQ ID NO: 23) speC_I163V_5 5′-ctgcttgttcatgaaggatcggcgaaagat- (SEQ ID NO: 24) 3′ speC_I163V_3 5′-ttcatgaacaagcagatcgcccaatttt-3′ (SEQ ID NO: 25) speC_E165G_5 5′-attcatggaggatcggcgaaagatgcgc-3′ (SEQ ID NO: 26) speC_E165G_3 5′-cgatcctccatgaataagcagatcgccc-3′ (SEQ ID NO: 27) speC_E165S_5 5′-attcattcaggatcggcgaaagatgcgc-3′ (SEQ ID NO: 28) speC_E165S_3 5′-cgatcctgaatgaataagcagatcgccc-3′ (SEQ ID NO: 29) speC_E165V_5 5′-attcatgtaggatcggcgaaagatgcgc-3′ (SEQ ID NO: 30) speC_E165V_3 5′-cgatcctacatgaataagcagatcgccc-3′ (SEQ ID NO: 31)

ODC mutants prepared by using the primers of Table 4 were purified according to the method of Examples 2 and 3 and the rates of putrescine conversion were measured. The results of measuring the rates of putrescine conversion of the prepared ODC mutants are given in the following Table 5.

TABLE 5 kcat Fold ODC enzyme K_(M) (mM) (sec⁻¹) kcat/K_(M) (sec⁻¹M⁻¹) (kcat/K_(M)) WT (wild-type) 1.5 1.6 1.1 × 10³ 1.0 I163G mutant 1.7 4.2 2.5 × 10³ 2.3 I163S mutant 1.5 7.4 4.8 × 10³ 4.4 I163V mutant 1.3 4.4 3.5 × 10³ 3.2 E165G mutant 3.0 5.6 1.9 × 10³ 1.7 E165S mutant 1.9 10.1 5.2 × 10³ 4.7 E165V mutant 1.4 10.9 7.6 × 10³ 6.9 I163A E165A 1.5 6.4 4.1 × 10³ 3.7 mutant I163S E165V 1.2 10.5 8.8 × 10³ 8.0 mutant I163A E165V 0.9 6.3 6.8 × 10³ 6.2 mutant I163V E165V 1.1 25.7 2.3 × 10³ 21.3 mutant

As shown in Table 5, when the amino acid residues 163 and 165 were introduced with single mutations of glycine (G), serine (S), and valine (V), respectively, replacement of the residue 163 with serine and replacement of the residue 165 with valine showed 4.4- and 6.9-fold increases in the kcat/K_(M) value compared to the wild-type, respectively. Based on this result, double mutations were introduced into the two residues, and their activities were examined. Surprisingly, not a double mutation of I163S and E165V combination, each single mutation showing the highest activity, but a double mutation of replacement of both 163 and 165 residues with valine showed a 21.3-fold increase in the activity compared to the wild-type.

Overall, the increased activities of the ODC enzyme mutants were attributed to an increase in a kcat/K_(M) value due to an increase in a kcat value, rather than a decrease in a K_(M) value, implying that the ODC enzyme is mutated to have a structure increasing a conversion rate into the product putrescine rather than a structure increasing the binding affinity of the substrate ornithine for the enzyme.

Example 5 Preparation of ODC Mutant Enzyme-Expressing Strain Using Ornithine as Substrate and Measurement of Putrescine Conversion

It was evaluated whether the ODC mutant enzymes in which mutations are optimized in Example 4 practically influence on conversion of ornithine into putrescine in a microorganism.

In detail, strains prepared by introducing E. coli having a DE3 genetic type with the prepared pET28a-speC mutant vectors were used to perform the experiment. Single colonies of respective strains were selected from LB plate media and inoculated into 3 mL of LB liquid medium (+kanamycin 50 μg/mL), followed by incubation at 37° C. and 200 rpm for 16 hours. The culture was re-inoculated into 25 mL of fresh LB medium (+kanamycin 50 μg/mL and 0.2% glucose), and incubated until OD₆₀₀ reached 0.5 to 0.6. Then, 0.5 mM IPTG was added to induce ODC (spec) expression, and incubated at 18° C. and 200 rpm for 20 hours. Then, centrifugation was performed to discard the supernatant and to collect cells. The cells obtained in the form of a pellet were resuspended in 1×M9 minimal medium (3.37 mM Na₂HPO₄, 2.2 mM KH₂PO₄, 0.86 mM NaCl, 0.94 mM NH₄Cl) to adjust an OD₆₀₀ value to 20. Additionally, 10 mM ornithine as a substrate and 0.5 μM PLP as a cofactor were added to a final volume of 10 mL. The reaction was allowed under conditions of 25° C. and 200 rpm with shaking, and sampling was performed over time. The concentration of the converted putrescine was measured by a method of quantifying putrescine using TNBS (Ngo T T, et al., Anal Biochem, 160: 290-293, 1987).

In the TNBS method, the supernatant obtained by centrifugation of the sampled culture was diluted 50-fold and used to perform the analysis. 1 mL of 4 N NaOH was added to 0.5 mL of the diluted sample, and then 2 mL of 1-pentanol was further added and mixed well. Centrifugation was performed at 2000 rpm for 5 minutes, and then 1 mL of the supernatant was added to a new tube containing 1 mL of 0.1 M Na₂B₄O₇ (pH 8.0), and these were mixed well. 1 mL of 10 mM TNBS was further added and mixed well, and 2 mL of DMSO was added thereto and mixed. Then, centrifugation was performed, and absorbance of the resulting supernatant was measured at 426 nm.

TABLE 6 0 hr 2 hr 4 hr Conversion Conversion Conversion put (mM) rate (%) put (mM) rate (%) put (mM) rate (%) WT wild-type 1.6 16 5.7 57 8.6 86 I163V mutant 1.5 15 7.5 75 9.8 98 E165V mutant 1.6 16 7.9 79 10.1 100 I163V E165V mutant 1.6 16 7.7 77 10.0 100

Putrescine conversion of the wild-type and 3 types of the ODC mutants were measured. As a result, the ODC mutants showed about 32% to 39% increased conversion rates of ornithine into putrescine in the sample collected at 2 hours compared to the wild-type. There was little difference in the conversion rate between the ODC mutations, and they showed no difference in the activity in which the purified ODC mutants showed a great difference in an in vitro experiment. However, the wild-type showed incomplete reaction even after 4 hours, whereas the mutants showed almost complete reaction within 4 hours. As a result, the increased activities of the ODC mutants were also confirmed in the enzyme conversion strains in vivo.

Example 6 Preparation of, Putrescine-Producing Strain Having ODC Mutant and Measurement of Putrescine Productivity

In order to examine whether the putrescine productivity is affected when the ODC mutants having increased putrescine conversion activity are practically introduced into the putrescine-producing strain, putrescine productivity was measured.

6-1. Preparation of Putrescine-Producing Strain Having ODC Mutant

Based on the Corynebacterium sp. microorganism (KCCM11240P) which has improved putrescine productivity by weakening the NCg11469 activity compared to the endogenous activity thereof, a mutant strain was prepared by changing the wild-type speC to the ODC (spec) having increased putrescine conversion activity in the chromosome.

The Corynebacterium glutamicum (KCCM11240P) strain having improved putrescine productivity is a strain disclosed in International Patent Publication No. WO2013/105827, and it was prepared using a Corynebacterium sp. microorganism (KCCM11138P) having putrescine productivity as a mother strain, disclosed in International Patent Publication No. WO2012/077995. In more detail, the strain was prepared by cloning the N-terminal and C-terminal regions of NCg11469 into a pDZ vector, based on the base sequence of NCg11469 gene of ATCC13032 strain, introducing the vector into a Corynebacterium sp. microorganism (KCCM11138P) having putrescine productivity by electroporation, and then plating the strain on a medium containing kanamycin (25 μg/mL), followed by selection. Successful chromosomal insertion of the vector was confirmed by selecting blue colonies on a medium containing X-gal (5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside). The primary chromosome-inserted strain was cultured in a nutrient medium, followed by spreading the diluted strain on a medium containing X-gal and no antibiotic and selecting white colonies which appeared at a relatively low ratio. Finally, an NCg11469 gene-deleted strain was selected by crossover. The final KCCM11138P ΔNCg11469 strain thus prepared is a putrescine-overexpressing strain having improved putrescine productivity compared to the mother strain KCCM11138P, the KCCM11138P ΔNCg11469 strain having a deletion of the gene encoding NCg11469, which is a protein involved in a pathway of decomposing putrescine into N-acetyl putrescine in cells.

In detail, DNA fragments of the ODC (spec) mutants prepared in Examples 2 and 4 was amplified using speC_start (BamHI)_5 and speC_stop (XbaI)_3 primers given in the following Table 7. Specifically, the prepared pET28a-speC mutant (I163S, I163V, I163S E165V) vectors as templates and two primers of speC_start (BamHI)_5 and speC_stop (XbaI)_3 given in the following Table 7 were used to perform PCR.

TABLE 7 Primer Primer sequence speC_start 5′-cgcggatccatgaaatcaatgaatattgc-3′ (BamHI)_5 (SEQ ID NO: 32) speC_stop 5′-gctctacattacttcaacacataaccgt-3′ (XbaI)_3 (SEQ ID NO: 33)

The gene fragments obtained by PCR and a vector pDZ were treated with restriction enzymes, BamHI and XbaI (37° C., 3 hours), and then the gene fragments of the speC mutants were inserted into the pDZ vector by a general ligation method, respectively. The recombinant vectors for chromosomal insertion (pDZ-speC_I163S, pDZ-speC_I163V, pDZ-speC_I163S E165V) thus prepared were confirmed by sequencing analysis.

To obtain strains in which the speC mutants were inserted into the chromosome, each of the prepared pDZ-speC_I163S, pDZ-speC_I163V, and pDZ-speC_I163S E165V recombinant vectors was transformed into the KCCM11240P strain by electroporation, and then spread on BHIS plate medium (37 g/L of brain heart infusion, 91 g/L of sorbitol, and 2% agar per 1 L+25 μg/mL of kanamycin).

Successful chromosomal insertion of the vector was determined by examining appearance of blue colonies on a solid medium containing X-gal (5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside). The primary chromosome-inserted strain was cultured in a nutrient medium with shaking (30° C., 8 hours), followed by serial dilution and spreading on the solid medium containing X-gal. Most colonies were blue, whereas white colonies appeared at a relatively low ratio. From the selected colonies, strains having speC mutants in the chromosome by secondary crossover were finally obtained. These strains were finally identified by sequencing analysis of the mutants. The identified strains were designated as KCCM11240P::speC_I163S, KCCM11240P::speC_I163V, and KCCM11240P::speC_I163S E165V. Of these, KCCM11240P::speC_I163S E165V was designated as Corynebacterium glutamicum CC01-0578 and deposited in the Korean Culture Center of Microorganisms (KCCM) on Jun. 10, 2013 with the accession number KCCM11425P under the Budapest Treaty.

6-2. Measurement of Putrescine Productivity of Putrescine-Producing Strain Having ODC Mutant

In order to examine the effect of ODC (speC) mutant on the putrescine productivity of the putrescine-producing strain, the strains prepared in Example 6-1 were evaluated for putrescine productivity.

In detail, the prepared strains were cultured in CM plate medium containing 1 mM arginine (1% glucose, 1% polypeptone, 0.5% yeast extract, 0.5% beef extract, 0.25% NaCl, 0.2% urea, 100 μL of 50% NaOH, 2% agar, pH 6.8 per 1 L) at 30° C. for 16 hours, and then a loop of cell culture was inoculated in 25 mL of titer medium of the following Table 8, and cultured with shaking at 200 rpm at 30° C. for 24 hours. All of the prepared strains were cultured with addition of 1 mM arginine in the medium during fermentation.

TABLE 8 Component Concentration/content (per 1 L) Glucose   8% Soybean protein 0.25% Corn steep solids  0.5% (NH₄)₂SO₄   4% Urea 0.15% KH₂PO₄  0.1% MgSO₄•7H₂O 0.05% Biotin  100 μg Thiamine hydrochloride 3000 μg Calcium-panthotenic acid 3000 μg Nicotinamide  3000 μug CaCO₃   5%

As shown in Table 9, each strain introduced with the ODC (spec) mutant having improved activity showed a 37% to 105% increase in the putrescine production at 24 hours.

These results show that the putrescine-producing strain having the ODC mutant is able to produce a high concentration of putrescine with respect to the sugar consumption compared to the known strain.

TABLE 9 12 hours Strain Put (g/L) Fold (%) KCCM11240P 1.3 100 KCCM11240P I163S mutant strain 2.7 205 KCCM11240P E165V mutant strain 2.7 193 KCCM11240P I163S E165V mutant strain 1.8 137

Based on the above description, it will be understood by those skilled in the art that the present invention may be implemented in a different specific form without changing the technical spirit or essential characteristics thereof. Therefore, it should be understood that the above embodiment is not limitative, but illustrative in all aspects. The scope of the invention is defined by the appended claims rather than by the description preceding them, and therefore all changes and modifications that fall within metes and bounds of the claims, or equivalents of such metes and bounds, are therefore intended to be embraced by the claims. 

1. A modified ornithine decarboxylase (ODC) protein having a mutation at one or more amino acid residues selected from the group consisting of an isoleucine at position 163 and a glutamic acid at position 165 from a N-terminus of ODC having an amino acid sequence represented by SEQ ID NO:
 1. 2. The modified ODC protein according to claim 1, wherein the glutamic acid at position 165 is replaced with alanine, glycine, serine, or valine.
 3. The modified ODC protein according to claim 1, wherein the isoleucine at position 163 is replaced with alanine, glycine, serine, or valine.
 4. The modified ODC protein according to claim 1, wherein the isoleucine at position 163 and the glutamic acid at position 165 are replaced with alanine-alanine, alanine-valine, serine-valine, or valine-valine, respectively.
 5. The modified ODC protein according to claim 1, having any amino acid sequence of SEQ ID NOs: 34 to
 57. 6. A polynucleotide encoding the modified ODC protein of claim
 1. 7. A vector comprising the polynucleotide of claim
 1. 8. A transformant transformed with the vector of claim
 7. 9. A method of preparing putrescine, the method comprising the step of reacting L-ornithine, a mixture containing L-ornithine, or an L-ornithine fermentation liquid with the modified ODC protein of claim
 1. 10. The method according to claim 9, wherein the modified ODC protein is a purified modified ODC protein or a microorganism fermentation liquid comprising the modified ODC protein.
 11. A recombinant microorganism which has improved putrescine-producing activity by changing to the modified ODC protein of claim 1 in a Corynebacterium sp. microorganism having putrescine-producing activity.
 12. The recombinant microorganism according to claim 11, wherein the Corynebacterium sp. microorganism is Corynebacterium glutamicum.
 13. A method of producing putrescine, comprising the steps of: (a) culturing a Corynebacterium sp. microorganism which has improved putrescine-producing activity by changing to the modified ODC protein of claim 1; and (b) recovering putrescine from a culture obtained in step (a).
 14. The method according to claim 13, wherein the Corynebacterium sp. microorganism is Corynebacterium glutamicum. 